Johnson 9.9-Power Loss

I have a 1998-Johnson 9.9.I have trouble with power loss.Motor starts out at 5600 to 5800 RPMs and 10 to 12 MPH.After about 1/2 hr to 1 hr running time the motor drops down to 5000 RPM and about 6 MPH.The plugs are new.The top sparkplug looks fine but the bottom is a little oily.

What do you think the problem might be?
I think it's a COIL going bad???????

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The scheduler at the mill asked me if I wanted to take a vacation next week.
I said "HELL NO!........MY boat is going in the shop Friday and get worked on.
"IF I can't go fishing it wouldn't be very good vacation." She laughed then I said-HOW ABOUT THE WEEK AFTER! 27 years in the mill and it's hard to get a summer vacation and I turn one down.
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